The dreaded sulfur smell

[Theswed]

I replaced my water heater in November/December last year and the last few months I noticed that dreaded sulfur/rotten egg smell. I haven't done a flush or hydrogen peroxide yet. I am worried about the safety of a aluminium/zinc anode. I am only left with a powered anode. Are aluminium/zinc anode safe? Could this been caused by me buying the wrong size tank? There was a 40 gallon orginally and it was actually more economical from me to buy a standard 50 gallon. Thanks